实现在属性更新时调用外部函数的Python类的正确方法

Question

I am currently trying to implement a python class that automatically synchronized with a NoSQL database with implicit buffering, quite to the image of the SLQAlchemy .

In order to do this, I need to track attribute updates issued by the user and, on each attribute update, call functions that keep that object in synchronization with the database or buffer.

What is the best way of doing this in Python? If it passes through __setattr__ and __delattr__ , how do I do it correctly, to avoid messing up with garbage collector?

Answer 1

One way to do it (the way I would recommend) is to use descriptors .

First you make a class for your properties, something like:

class Property:
    def __init__(self, *args, **kwargs):
        #initialize the property with any information it needs to do get and set
    def __get__(self,obj, type=None):
        #logic to get from database or cache

    def __set__(self,obj, value):
        #logic to set the value and sync with database if necessary.

And then in your class entity class you have something like this:

class Student:
    student_id = Property(...)
    name = Property(...)
    classes = Property(...)

Of course in practice you may have multiple Property types. My guess is that SQLAlchemy does something like this, where Column types are descriptors.